This invention relates in general to holding valves for hydraulically actuated devices and in particular to a pilot actuated override mechanism for such a holding valve.
Hydraulically operated machines are frequently used in construction projects and other activities. One commonly known hydraulically operated machine is the backhoe. Backhoes are used both for lifting loads upwardly above the ground and for digging downwardly into the ground. To accomplish this, backhoes are provided with articulated shovel assemblies which are moved by hydraulic operating systems. Such systems are provided with one or more valves which are manipulated by an operator to cause appropriate movement of the shovel assembly. For example, the shovel assembly can first be used to dig downwardly to excavate the soil where a pipeline is to be laid. Then, having removed the soil, the shovel assembly can be used to lower a length of pipe into the hole and to support it adjacent to a previously laid pipe so that the two pipes may be welded together.
The hydraulic operating systems of backhoes and similar machines typically include a source of pressurized fluid which is selectively connected to a cylinder containing a movable piston. The piston sealingly engages the inner surface of the cylinder, dividing it into first and second chambers. As is well known, by connecting the source to supply pressurized fluid to the first chamber and by simultaneously venting the second chamber, the piston can be moved in one direction relative to the cylinder. Conversely, by connecting the source to the second chamber and by venting the first chamber, the piston can be moved in the opposite direction relative to the cylinder. Lastly, by preventing fluid within both of the cylinder chambers from escaping therefrom, the piston can be locked in a predetermined position within the cylinder. The piston is connected by a rod to the movable member of the machine (i.e., the shovel assembly of a backhoe) for movement therewith.
In order to control the flow of pressurized fluid to and from the cylinder chambers, a control valve is usually connected between the source of pressurized fluid and the cylinder chambers. A conventional four-way valve is frequently employed for this purpose. The four-way valve usually has three operating positions. In its first position, the four-way valve connects the source of pressurized fluid to the first chamber and vents the second chamber, causing the piston to move in one direction. In its second position, the four-way valve connects the source of pressurized fluid to the second chamber and vents the first chamber, causing the piston to move in the opposite direction. In its third position, the four-way valve prevents fluid within both of the chambers from escaping therefrom, causing the piston to be locked in a predetermined position.
In most hydraulically operated machines which perform lifting functions, a holding valve is connected in the hydraulic lines extending between the four-way valve and the cylinder, typically directly adjacent to the cylinder. The holding valve is a well known device which performs several functions. First, the holding valve reliably seals the chambers of the cylinder when it is desired to maintain the load at an elevated position for a lengthy period of time, since the four-way valve is sometimes prone to leakage and consequent movement of the load. Second, the holding valve carefully modulates the rate at which hydraulic fluid flows from the cylinder chambers, thereby regulating the speed at which a heavy load is lowered. Third, the holding valve provides a static overload relief function, allowing excess pressurized fluid to escape from the system before causing damage. Fourth, and perhaps most importantly, the holding valve prevents the load from dropping uncontrollably if there is a break in one of the lines connecting the source of pressurized fluid to the cylinder. If this occurs, the holding valve prevents any hydraulic fluid from flowing in or out of the cylinder chambers, thereby locking the piston within the cylinder and preventing the load from falling.
Since most backhoes can perform both lifting and digging functions, their hydraulic operating systems are generally provided with holding valves for the reasons described above in connection with lifting machines. When the backhoe is used to perform a digging operation, however, there is little need for such a holding valve, since the shovel assembly is not raised a significant height above the ground. Unfortunately, when it is desired to use the shovel assembly to dig below ground level, the holding valve can have an undesirable effect on the operation thereof. The flow rate modulation of the hydraulic fluid flowing through the holding valve can limit the speed at which the shovel assembly is operated to dig into the ground. As a result, the shovel assembly may undesirably start and stop abruptly during use, as opposed to a preferable smooth motion. This is particularly so when an experienced backhoe operator attempts to move the shovel assembly rapidly through a digging cycle. Therefore, when the shovel assembly of a backhoe is operated in a digging cycle, it would be desirable to override the operation of the holding valve described above to permit a smoother digging motion to occur.